| Microbial redox cycling of iron (Fe) plays a central role in controlling terrestrial element partitioning, with broader impacts on subsurface contaminant mobility and the development of geochemical cycles on Earth. This dissertation explores the capacity of native microbial communities to oxidize and reduce Fe in terrestrial circumneutral-pH environments by coupling environmental sampling campaigns to bench-top experiments.;Microbial Fe-oxide reduction can result in magnetite production, potentially explaining magnetite in Precambrian sedimentary rocks. The only modern analog occurs in sediments at the Bay of Vidy (Lake Geneva, Switzerland), where in situ magnetite formation is directly associated with microbial reduction. Geochemistry and isotope composition of Fe in these sediments was investigated. Despite extensive microbial reduction, very little Fe isotope variation was observed, indicating Fe isotope homogeneity is not sufficient to rule out a biological mechanism for magnetite formation.;Microbial oxidation of solid-phase Fe(II) at circumneutral pH is a key pathway in controlling element partitioning and contaminant stability in modern terrestrial environments. Experimental reactors were constructed with sediment from a contaminated subsurface environment (Hanford 300 Area, Richland, WA) to determine how native microbes responded to oxidant flux. Endogenous microbial communities rapidly responded to chemical oxidant input (O2 or NO3) concurrently stabilizing. Uninoculated oxic reactors showed significant Fe(II) oxidation, enhancing our understanding of the capacity for native microbes in subsurface sediment to maintain redox state.;A second series of reactors explored the capacity of native microbes to oxidize solid-phase Fe minerals. The first microbial cultures capable of aerobic pyrite (FeS2) oxidation at circumneutral pH were recovered. These enrichment cultures demonstrated growth tied to pyrite oxidation and sulfate generation for over one year of repeated transfers. Aerobic pyrite oxidation has been proposed to be a dominant pathway in Precambrian sulfur cycling. The mineralogical, microbiological, and geochemical changes observed in these experiments shed light on the role of microbes in sulfur cycling in ancient and modern subsurface environments.;Finally, my Teaching and Learning Portfolio, the capstone requirement for the Delta Program's Certificate in Research, Teaching, and Learning, is included to highlight some of the broader impacts of my graduate career. |
